Panel displays are commonly used in electronic products. It is known to provide panel displays based on electrophoretic effects. Electrophoretic effects comprise charged particles dispersed in a fluid or liquid medium moving under the influence of an electric field. As an example of the application of electrophoretic effects, displays may use charged pigment particles dispersed and contained in a dye solution and arranged between a pair of display electrodes. The dye solution in which charged pigment particles are dispersed is known as “electrophoretic ink” or “electronic ink.” A display using electrophoretic ink is known as an electrophoretic display (“EPD”). Under the influence of an electric field, the charged pigment particles are attracted to one of the pair of display electrodes. In response, desired images are displayed.
In recent years, EPD technology has been introduced for use in flat panel displays. FIGS. 1A and 1B illustrate this technology using microcapsules filled with electrically charged white particles suspended in a pigmented oil. For example, FIG. 1A illustrates one implementation in which the underlying circuitry controls whether white particles are at the top or bottom of the capsule. In this example, if the white particles are at the top of the capsule, the display appears white to the viewer. On the other hand, if the white particles are at the bottom of the capsule, the viewer sees the color of the oil, as illustrated in FIG. 1B. As a result, the use of microcapsules allows the display to be constructed using flexible plastic sheets, as well as glass.
One feature of EPD technology is that the pixels are bistable. That is, the pixels can be maintained in either of two states without a constant supply of power. Another feature of EPD technology is that particles in an EPD panel move in different directions according to control voltages, in order to display different colors. As a result, EPD panels have a response time which is slower than those of other types of flat panel display.
One application of EPD technology, the electronic paper display device, is being developed as a next generation display device to replace liquid crystal display devices, plasma display panels, and organic electro-luminescent display panels. In particular, electronic paper display panels using “electronic ink” are expected to be a replacement, in certain applications, for existing print media such as books, newspapers, magazines, or the like. E Ink Corporation is an example of a company active in development of such displays.
The electronic paper display device is well suited for use as a flexible display device because the device can be constructed to include a flexible substrate. For example, an electronic paper display device constructed to include a substrate of flexible material, may have advantages in terms of flexibility, simplicity, and reliability. Development of the electronic paper display device may also lead to construction of paper-thin reflective displays without use of a backlight, resulting in very low power consumption.
More generally, however, available methods for driving EPD panels have a relatively long response time. For example, data is displayed depending on the motion of particles. As a result, it is not suitable for displaying images that embody moving images. Also, EPD panels also have limitations in representing full color and gradation.
Another difficulty in the application of EPD technology is that the driving schemes used with traditional flat display panels, such as liquid crystal displays (LCD), do not produce the same performance when applied to drive an EPD. Two reasons for this are described below.
First, EPD and LCD applications have respectively different display response times. For example, when a display panel displays video (i.e., moving) images, the pixel data of different image frames change at a rate of tens of times per minute. In this condition, the brightness of pixels is controlled by a driving circuit, by changing levels of the driving voltages applied to the pixels. There is a time period for the driving circuit to hold the levels of the driving voltage. In LCD display applications, the driving circuit is required to hold the levels of the driving voltages over a time period in the range of 10 ms, depending on display resolution and frame frequency. However, the hold time required by the EPD is relatively an order of magnitude longer than that required for a traditional display panel, such as LCD.
Second, because EPD applications have a much longer response time, an EPD may have a pixel layout and driving methods different from those implemented for a traditional flat display panel, such as the LCD. In an LCD application, pixels are arranged in rows and columns. This arrangement is known as a dot-matrix pixel layout. Each and every row or column is activated sequentially. That is, the rows or columns are activated one at a time, in a scanning manner. Each pixel in a row or column has its own electrode for receiving a driving voltage. When each row or column is activated, all pixels present in the row or column are updated by the same control unit. Display apparatuses for driving displays with the dot-matrix pixel layout are divided into two types: passive matrix (PM) type and active-matrix (AM) type.
In the passive matrix (PM) display, a matrix of electrically-conducting columns and rows are orthogonally arranged to form a two-dimensional array of picture elements, i.e., pixels. Positioned between the orthogonal column and row lines, thin films of display material are activated to display black or white colors. This is achieved by applying electrical signals directly to the designated rows and columns.
In contrast, an AM display panel, consists of display pixels that have been deposited or integrated with a thin film transistor (TFT) array to form a matrix of pixels that displays images upon electrical activation. A TFT backplane acts as an array of switches that control the connection of applied image signals to each pixel. The TFT array continuously determines if and when signals are applied to the pixels, resulting in a scan of all pixels and in display of a corresponding image on a panel.
FIG. 2 is a schematic view depicting display driving electronics system for an AM TFT LCD 208 with K columns by L rows. As shown in the figure, if there are K pixels located in the horizontal direction, K channels of source drive units (SDUs) 202 are required for driving K columns of pixels of the LCD 200. In the vertical direction, a gate driver 206 is employed to drive a voltage on each of L scanning lines sequentially, to turn on and off TFT's 208 of the pixels on each row for sampling and holding the voltage level outputted by the SDU's 202. As a result, each row is activated sequentially by the gate driver 206 in repeated scanning cycles.
For both AM and PM type displays, in order to display a full image, each row of the display must be updated in 1/N of the frame time needed to scan the entire display, where N is the number of rows in the display. For example, in order to achieve a 220-row display image, the pixels must be driven to the required color in 1/220 of the entire frame time. The scanning speed must be sufficiently fast, such that the sequentially activated elements appear to the human eye as being activated simultaneously, thus allowing for a proper and consistent image, as perceived by the user. However, this requires an updating time for a single row in the range of 75 μs with a frame frequency of 60 Hz. Characteristics of the LCD panel enable such fast display-updating speed. However, because EPD applications require a much longer response time in order to update a pixel (which may be as long as seconds), the above scanning scheme may lead to a very slow image refresh rate for EPD applications. This disadvantage may lead to a non-user-friendly interface in applications including or requiring interaction between a user and a driver IC.
An example of a scan-driving PM-type EPD is described in U.S. Pat. No. 4,947,157 to Di Santo et al. (“Di Santo”). Di Santo discloses a driving apparatus for an electrophoretic display. FIG. 1 of Di Santo is reproduced herein as FIG. 3A. With reference to FIG. 3A, the display of Di Santo includes a cathode electrode 11, which is one of a series of lines arranged in a horizontal X direction. Associated grid lines 12 appear to run in the Y direction and are insulated from the cathode electrode 11 by insulating layer 13. The cathode electrodes 11 and the gridlines 12 form an X-Y matrix. An anode electrode 15 overlies an electrophoretic dispersion 16 between the cathode electrode 11 and anode electrode 15 and contains a plurality of submicron pigment particles. When a potential is applied between an X and Y point indicative of a pixel accelerating particles (e.g., particles 17 and 18), in the vicinity toward the anode where they remain until bias is reversed.
As shown in FIG. 3B, Di Santo discloses the X-Y matrix consists of the cathode lines which are arranged in the horizontal plane and the grid lines which are perpendicular to the cathode lines and which are insulated from one another. Each cathode line has a suitable driving amplifier circuit shown in modular form and indicated by reference numerals 40, 41, 42, and 43. Each grid line has a suitable driving amplifier referenced by modules 50, 51, 52, and 53. The driving signals for the grid and cathode lines are obtained by X-driving generator 60 and Y-driving generator 61. In a display data write mode, pixels are updated row by row. The cathode lines which have pixels to be written are placed at zero potential, one by one, in a line-scanning scheme while non-writing cathode lines are placed at positive voltage (potential). When a cathode line is selected, writing grid lines are operated at a high potential and non-writing grid lines are operated at the low or zero potential. As a result, pixels are updated row by row.
For example, in order to update pixels 70 and 80 in FIG. 3B, cathode line 20 is set to zero potential while other cathode lines are kept at a positive potential. Pixel 70 is then updated by applying a positive voltage (potential) to the corresponding grid line 30 while applying a low or zero potential to other grid lines. After the updating of pixel 70 is finished, cathode line 23 is set to zero potential while other cathode lines are kept at a positive voltage (potential). Pixel 80 is then updated by applying a positive voltage to grid line 32, while applying a low or zero potential to other segment lines. This procedure is repeated until all rows are updated.
Although the scan-driving scheme of Di Santo achieves a high display resolution, it may result in a slow image-update speed. In order to refresh a display, the pixels in the array have to be updated row by row. Each and every row which has pixels to be updated is activated and updated sequentially, one at a time, in a scanning manner. When a row of pixels is selected to change or update data information, the update of this row cannot be initiated until the changes in a previous row have been completed. Therefore, the minimum refresh time required for a display in this scanning scheme is a product of the number of rows which have pixels to be updated, multiplied by the update time required for an individual pixel.
As a result, scan-driving type EPD may be an undesirable choice for applications requiring reliable human-machine interface, because the scan-driving type EPD responds slowly to user inputs. Furthermore, due to the slow image update speed, i.e., updating all pixels together in one row and having all pixels refreshed after one frame, in a prior art dot-matrix pixel layout arrangement, EPD applications cannot support high display resolution or motion-type image quality.
One possible solution for overcoming such shortcomings, when the EPD is used in applications that do not have many pixels but which require a real time response, is use of a segment drive (or direct drive) such that all pixels are updated at the same time. FIG. 4 illustrates a block diagram of a conventional display driving system in which all pixels are driven at the same time. As shown in FIG. 4, all pixels of a display panel 406 are controlled by a waveform generating unit 402 coupled to segment cells 404a, 404b, . . . , 404m, and are updated at the same time. Without the scanning process, the refresh time required for the display 406 is just the update time required for one pixel.
An example of a segment display driver is a 40 segment static LCD driver chip V6108 manufactured by EM Microelectronic. FIG. 5 is a block diagram of this driver. A 40 bit shift register and the 40 latches correspond to the waveform generating unit 402 of FIG. 4, while voltage level shifters LS correspond to the segment cells 404 of FIG. 4. Each LS drives a segment pixel, e.g., SEG 1-SEG 40 in the display panel 406. The latches update the output signals to all level shifters 40 LS at the same time under control of various signals.
However, use of a conventional segment display driver to drive an EPD panel may have disadvantages. For example, when a waveform is applied to update the display in an EPD panel, another update can only be initiated after the completion of the previous update for the entire display. Although the display refresh time has been decreased to only the update time of one pixel, it may still be as long as several seconds in some cases.
There are limitations on the use of a segment display driver. For example, all segment cells are driven to provide outputs waveforms at the same time and are fixed by design of the driver IC. Regardless of whether data is changed or whether only a single pixel needs to be updated, all segment cell units would accordingly output driving waveforms at a fixed time. Second, because of this limitation, programmers or users can only update data after a previous input has been displayed on the EPD panel. This limits flexibility, since it does not allow programmers or users to update data at different times or more arbitrarily.
One solution to such limitations is illustrated by a display segment driving system 600 shown in FIG. 6. In system 600, a separate waveform generating unit 602, e.g., 602a, 602b, . . . , 602m, is provided for each pixel. A separate segment cell 604, e.g., 604a, 604b, . . . , 604m is respectively provided for each generating unit 602. Each of the segment cells 604 are coupled to a display panel 606. However, because every pixel is driven by a separate waveform generator 602, the display update of a second input can be started immediately regardless of whether or not the display update of a previous input has been completed. Therefore, an instant display response can be achieved. However, such an arrangement requires a great amount of additional circuit area, which may not be feasible for certain applications.
While problems with driving bistable displays have been described with reference to EPD panels, bistable stable displays may be constructed using other technologies. For example, Nemoptic is an e-paper display company that develops bistable liquid crystal displays. The above described problems with driving bistable displays need to be addressed regardless of the technological basis for the bistable display's construction.
Thus, there is a need for a method and system directed to improving driving of a bistable display.